The invention concerns a microporous isotropic cellulose nitrate ("CN") membrane on a polyester support which finds application in the production of sorbent reactants, in particular, test strips for use in analytical chemistry and medicinal diagnostics, and to a process for the manufacture of such a membrane.
In the case of composite membranes reinforced by film supports, special challenges arise in regard to their required characteristics. The membrane must be durably adhered to the film support and must not separate therefrom during further processing or in the course of its use. Ample hydrophilicity of the membrane must be maintained throughout both storage and during the course of use. Unless such characteristics are met, reproducibility of analytical and diagnostic results cannot be guaranteed. Furthermore, in order for analyses and diagnostic examinations to proceed rapidly, the supported membrane must exhibit a high lateral migratory velocity for the liquids tested. "Lateral migration velocity" means the speed with which a liquid introduced to the surface of a membrane spreads in a direction generally parallel to the support film, and is expressed in units of cm/min or mm/min.
Composite supported membranes used as sorbent test strips in analytic and diagnostic procedures are known. German Patent No. 34 07 359 describes a supported membrane comprising an asymmetric membrane upon a macroscopically smooth polyester film made by coating the polyester support with a polymeric casting solution comprising polyamides, polyether carbonates, polyacrylic nitriles or polyurethanes, whereby the polymeric coating is deposited on the support by phase inversion. A primary drawback of such a membrane lies in the unsatisfactory adhesion of the membrane to the support. Although German Patent No. 40 09 186 discloses a method of achieving a more lasting adhesion between an asymmetric polyurethane membrane and a support, the method comprising the use of a special polymer blend support having a roughened surface treated with an anti-static solution, this approach is relatively complex and costly. Another serious drawback of asymmetric membranes is that the asymmetric structure leads to a lateral migration velocity gradient for liquids introduced to the membrane, whereby liquid moves faster in the zone of the larger diameter pores (those adjacent the support) than in the zone of the smaller diameter pores (those on the opposite side of the membrane). Because of this gradient, use of such a supported membrane as a sorbent test strip often yields blurred, inexact and difficultly reproducible indications for the material being tested.
Because of its good adsorption characteristics, particularly with respect to substances containing protein, CN is a preferred membrane material for sorbent reactants. However, no durable adhesion of a pure CN membrane on a polyester support may be expected from a mere casting of the membrane on the polyester support. European Patent No. 0 421 235 describes an approach to securing a CN membrane to a support, namely, a porous laminated CN membrane is glued to a polyester support film by a layer of adhesive. The chief disadvantage of such a membrane is that with adhesives of this type, as storage time increases, a loss in adsorption potential and of the hydrophilicity of the membrane layer can occur, because the adhesive migrates into the membrane and reacts with surfactant which is also present in the membrane. Moreover, the fabrication of such laminated membranes and gluing of the same to the supports is a costly multi-step process. Lamination of CN membranes by merely using heat and pressure is not possible both because no durable adhesion is achieved and because it causes the CN membrane to become partially compacted, resulting in a substantial loss in the membrane's lateral migration velocity.
A principal object of the present invention, therefore, is the provision of a composite microporous CN membrane on a polyester support wherein the CN membrane is bonded durably to the support and which exhibits no lateral migration gradient, all without the use of heat, pressure or adhesives. A further object is to provide an economical procedure for the production of such a composite membrane. These objects and others will become apparent upon consideration of the invention, which is summarized and described in detail below.